Learn Dutch: Het fruit is niet duur. - The fruit is not expensive.

Geen is used to negate an indefinite noun (like saying "not a" or "no" in English). In this sentence, we are not negating the noun; we are negating the adjective duur (expensive). Whenever you negate an adjective, you must use niet.

How would I use geen with these words?

If you wanted to say "No fruit is expensive," you would use geen because you are negating the noun: Geen fruit is duur. But here, to say "The fruit is not expensive," you use niet to negate the adjective.

Why is it het fruit and not de fruit?

Fruit is a neuter noun in Dutch (a "het-woord"). There is no specific rule for this, so you simply have to memorize that it takes het instead of de.

Why doesn't duur get an -e at the end?

Adjectives in Dutch only get an extra -e when they are placed directly in front of a noun. When the adjective comes later in the sentence after the verb, as it does here, it always stays in its basic dictionary form.